An erasable phase-change optical disk utilizes a phase change between the crystalline and amorphous states of a recording layer to accomplish the functions of writing and erasing. The working principles of the erasable phase-change optical disk will be introduced in conjunction with typical prior art references hereinafter for a better understanding of the present invention.
A typical erasable phase-change optical disk is shown in FIG. 1, which comprises a phase-change recording layer 2 interposed between an upper dielectric layer and a lower dielectric layer 3 on a substrate 1, a reflective layer 4 on the upper dielectric layer, and a plastic protection layer 5 on the reflective layer 4. A suitable material for making the dielectric layers 3 is SiO2—ZnS. The substrate 1 may be formed of polymethyl methacrylate, polycarbonate or a glass. Suitable materials for forming the reflective layer 4 include Au, Cu, Al, Ni, Cr, Pt, Pd and an alloy thereof.
The currently used erasable phase-change optical disks utilize a chalcogenide material based on Te or Se as the recording layer. When a region of the recording layer is subjected to a rapid heating to a molten state upon irradiation of a focused laser beam with a high power short pulse modulation, the region will be conductively quenched by the adjacent layers (e.g. the dielectric layers and reflective layer) to an amorphous state, so that a recording mark is formed. The amorphous recording mark has a reflectance lower than that of the blank crystalline region (for some special alloys the reflectance of amorphous recording mark is higher), and the difference in reflectance is used for reproduction of signals. A medium power and long pulse laser beam is used to erase the recording mark, which resumes the blank crystalline region by heating to a temperature between its melting point and crystallization point.
The chalcogenide material was first used as the phase-change recording layer by S. R. Ovsinsky, et al. in U.S. Pat. No. 3,530,441, wherein thin films of Te.sub.85 Ge.sub.15 and Te.sub.81 Ge.sub.15 S.sub.2 Sb.sub.2 produce a reversible phase-transition according to irradiation with high energy density light such as the laser beam. Thereafter, most of the research works have concentrated on the chalcogenide materials, for examples GeTe, InSe, InSeTI, InSeTICo, GeSbTe, GeTeSn, GeTeAs, GeTeSnAu, InTe, InSeTe, InSbTe, and SbSeTe, etc. all pertain to the chalcogenide material. Among them, the series of GeSbTe alloys developed by Matsushita Electric Industrial Co., Ltd., Japan, in U.S. Pat. Nos. 5,233,599; 5,278,011; and 5,294,523 are the most promising ones. The details of these patents are hereby incorporated by reference in their entirety.
In the GeTe—Sb2Te3 pseudo-binary alloy system, three intermetallic compounds exist in between the GeTe and Sb2Te3 terminal compounds. They are, in sequence, Ge2Sb2Te5, GeSb2Te4 and GeSb4Te7. Yamada et al. of Matsushita heavily studied these pseudo-binary alloys, and disclosed their results in the Journal of Applied Physics 69(5), pp. 2849 (1991). They found that the laser-induced crystallization time for the amorphous film can be smaller than 100 ns that decrease with increasing Sb2Te3. When the composition deviate the pseudo-binary line, the crystallization time rapidly increases. Crystallization temperature, around 200° C. lowers with increasing Sb2Te3. A metastable FCC structure forms at the onset of crystallization, it then converts to a stable HCP structure.
With decreasing spot size of the laser while constant increase of recording density, new recording media with faster and faster crystallization speed is drastically needed to improve data transfer rate. Scientists in Philip found that the compositions at around Sb70Te30 in the same GeSbTe system are characteristic of superior crystallization capability (see the Japanese Journal of Applied Physics 40, p. 1592 (2001)). These compositions show low nucleation rate while high growth rate, hence were called growth driven or fast growth type of optical recording media. Traditional GeSbTe compositions, such as Ge2Sb2Te5, belong to nucleation driven type of optical media, and is characteristic of large amount of nuclei formation together with grain growth. The crystallization speed depends on the time required for nucleation and growth. While for growth driven type of media, crystallization proceeds with the enlargement of existing nuclei at the peripheral of amorphous recording spots. Time for crystallization depends on only the time for grain growth that favors the ever smaller recording bits.
The growth driven materials in use are, among others, AgInSbTe of Ricoh, InGeSbTe of Phlip, those are compositions near Sb70Te30, with a modification using Ag, In, and/or Ge to tailor the crystallization behavior. The present invention proposes a new alloy system based on GaSbTe ternary alloys possessing high growth driven crystallization speed and is suitable for the media of high recording density using blue lasers.